Metallic electrodes are used in a wide variety of electrochemical applications, and they have been so used for very many years. For example, a metallic electrode may be used as an anode in the cathodic protection of a metallic structure, and the anode may be of the sacrificial type or of the impressed current type. A metallic electrode may be used in a wide variety of electrolytic cells. For example, a titanium electrode coated with a suitable electrocatalytically-active material may be used as an anode in an electrolytic cell in which chlorine and caustic alkali, or alkali metal hypochlorite or alkali metal chlorate, are produced by electrolysis of aqueous alkali metal chloride solution. In the electrolysis, the metallic cathode may, for example, be of steel or of nickel or of a nickel alloy. Other applications of metallic electrodes include use in fuel cells, in metal winning, in electroorganic synthesis, and in metal plating.
In some electrochemical applications, metal electrodes suffer from certain disadvantages, and in particular metal electrodes may tend to be chemically attacked by the medium in which the electrode is used, indeed, the metallic electrode may be consumed at an unacceptably high rate. For this reason, and for other reasons, e.g., the expense and the relative ease of manufacturing the electrodes, electrodes made of non-metallic materials have been developed in recent years. For example, electrodes made of magnetite have been developed which are dense and hard and which, because the iron in the electrode is in a highly oxidised state, are very resistant to further oxidation when used as an anode. Indeed, the magnetite is virtually inert in many applications and at least in this respect functions in the manner of a precious metal. Magnetite electrodes have been developed for use as anodes in cathodic protection applications. Electrodes may also be made of ferrite. Another non-metallic electrode which has recently been developed for a wide variety of electrochemical applications, including use as an anode in cathodic protection applications, is a ceramic-like electrode formed of a solid coherent bulk titanium oxide having the general formula TiO.sub.x, where x is a number in the region 1.67 to 1.9. An electrode of this latter type is described in European Patent No. 47595 and in U.S. Pat. No. 4,422,917. The electrode, which is sold under the trade name `Ebonex` by Ebonex Technologies Inc., has a particularly desirable combination of properties, namely relatively high electrical conductivity, a high chemical resistance, and good anodic and cathodic electrochemical stability. The electrode is particularly stable in acid environments. Other electrodes which consist of titanium oxide are described, for example, in GB Patent No. 1443502 in which there is described an electrode which may consist of bulk TiO.sub.x in which x is in the range 0.25 to 1.50, and is preferably 0.42 to 0.60.
Although non-metallic electrodes generally have high chemical stability, such electrodes generally have electrical conductivities which are not as great as the electrical conductivities of metallic electrodes, and these relatively low electrical conductivities do lead to some problems in use. Thus, as a result of the relatively poor electrical conductivity, the current distribution in a non-metallic electrode may be relatively poor with the current flow through the electrode progressively decreasing with distance from the point of electrical connection, and indeed, in a non-metallic electrode of relatively large size, there may be a very low or negligible flow of current at a point remote from the electrical connection. The electrical current will of course flow along the path of least electrical resistance, and the current may pass into the environment adjacent to the electrode rather than distribute itself throughout the electrode. Such relatively poor current distribution can itself lead to problems in that there may be localized areas of high current density on the surface of the electrode which may result in damage to the electrode and a reduction in the useful life of the electrode.
Various solutions to this problem of poor current distribution in non-metallic electrodes have been proposed. In general, these solutions involve laminating the non-metallic electrode to, or coating the non-metallic electrode with, a metal of low electrical resistivity. In WO No. 8303264-A there is described a tubular magnetite electrode for use in impressed current cathodic protection which has an internal coating of electrically conductive metal alloy to which an electrical connection is made. The coating may be of copper, lead, tin or aluminium, or of alloys thereof. Similarly, Swedish patent No. 7714773 describes a tubular magnetite anode which has an internal copper lining. A metal oxide electrode for use in cathodic protection in the form of a hollow tube which has a coating of an electrically conductive metal or alloy is also described in U.S. Pat. No. 4,486,288. The internal coating or lining may be applied to the tube by electroplating, but we have found that such electroplating is technically rather difficult to effect and may result in a poorly bonded coating or lining. Furthermore, if the coating or lining is to act as an efficient current distributor, a relatively thick coating or lining is required and such a thick coating or lining may take a substantial time to apply.
In GB Patent No. 2114158 there is described a tubular electrode which consists of a sintered mixture of ferrite and one or more oxides of Mn, Ni, Co, Mg, Cu, Zn, or Cd. The tube has one end open and one end closed and a metal member, which may be, for example, a bar of stainless steel, inserted into the tube and fixed to the tube by means of an electrically conductive material.
The present invention relates to an electrode which comprises a non-metallic material of relatively low electrical conductivity, which is simple to construct, and which in use shows excellent current distribution over the surface of the electrode.